Analog to digital converters, also referred to as encoders, are commonly designed with a feature known as second order integration to improve the signal to noise ratio of the encoders, particularly of the over-sampled type. One common form of an over-sampled encoder is a delta-modulator in which the level difference between an input signal and a reconstruction of that input is measured with a voltage comparator to produce a digital output. This output is fed back to the reconstruction circuit, normally an integrator, to create a signal to match the input. The level difference between the input and the reconstruction of the input is known as the quantizing error or noise.
The quantizing error is a noise component which has frequency contributions from DC to one-half the sampling frequency. With delta-modulators, the sampling frequency is often one to three orders of magnitude above the highest frequency of interest. For example, for telephone voice encoding, the frequency band of interest might be 300 hertz to 3,000 hertz, while the sampling rate is 512,000 hertz. The quantizing error that occurs in the band of interest is undesirable because it reduces the usable dynamic range and hence affects the SNR. This quantizing error can be reduced in the band of interest by using second or higher order integration.
Higher order integration reshape the spectrum of the quantizing error signal by filtering and attenuating those spectral components that occur above the band of interest so that the voltage comparator will measure level differences between the input and the band of interest frequency components of the reconstruction signal. The voltage comparator's sampled output, the digital bits, will have a more accurate measure of the then quantized errors in the band of interest and will therefore have a higher SNR in the band of interest.
Many variations of the process used in higher order integration are employed in the art. For example, one variation is described in U.S. Pat. No. 4,509,037. The encoder described in that patent employs a spectrum tilter, a one bit analog to digital converter, a sampling circuit, and an internal decoder. An analog input signal and an internal analog signal from the internal encoder are summed to provide an analog dither, which is essentially an internal error signal. The analog dither signal is tilted by the spectrum tilter and is provided to the one bit analog to digital converter which generates a digital signal. The sampling circuit receives a digital signal from the analog to digital converter and generates a digital output which is fed back to the internal decoder. The spectrum tilter comprises at least three integrator circuits and a clipping circuit connected in parallel to two of the three integrator circuits. The three integrator circuits tilt the frequency spectrum of noise above the maximum frequency of interest and the clipping circuit prevents the encoder from becoming unstable.
The level difference techniques described in the prior art are not applicable to a new class of analog to digital converters or encoders as represented by co-pending application Ser. No. 566,314 filed Dec. 28, 1983 now U.S. Pat. No. 4,683,456 of which the present application is a continuation-in-part. The analog to digital converter described in the foregoing application utilizes phase shift principles in the generation of digital bits. More particularly, two high-frequency digital signals are generated with the average frequency of each signal being constant and the frequencies bearing a harmonic relationship one to the other. A representation of an analog condition is applied to cause a shift in phase in at least one of the digital signals. The digital signals are then compared to obtain a measure of the phase shift which is utilized to adjust the phase of the phase shifted signal toward its original phase condition. As a result of the measure of phase shift, there is generated a digital function which is representative of the amplitude of the adjustment utilized to return the phase shifted signal toward its original phase condition. Like the level differences described in the prior art, the phase differences of the analog to digital converter described above include quantizing errors which have frequency components in the frequency band of interest. Accordingly, in order to enhance the operation of the analog to digital converter above described, there arose a need to provide an entirely new approach to solve the problem of distortion introduced by quantizing noise or error.